Series and parallel circuits: explain the design and use of dc series circuits for measurement and testing purposes

The idea

A dc series circuit has one continuous path, so the same current flows through every component and the supply potential difference is shared between them. This makes series circuits useful for measurement and testing: an ammeter is connected in series so that the current being measured passes through it, and a variable resistor in series can control that current. A test circuit might contain a cell, a switch, an ammeter, a component under test and a variable resistor. The switch makes and breaks the circuit safely, the ammeter monitors current, and the variable resistor allows the potential difference across the component to be varied so that current-potential difference characteristics can be investigated. A voltmeter is connected in parallel with the component whose potential difference is being measured, because it must compare the two points across that component.

What earns the marks

• A dc series circuit provides a single path, so the current is the same through each component, which allows an ammeter to measure that current directly.
• An ammeter is connected in series with the component under test so that the current to be measured passes through the meter.
• A variable resistor connected in series can change the total resistance and therefore control the current and the potential difference across a component.
• A switch in series allows the circuit to be closed or opened safely during testing.
• A voltmeter is connected in parallel with the component being tested so that it measures the potential difference across that component.
• Series test circuits are used to investigate how current through a component depends on the potential difference across it, for example for a filament lamp or a resistor.

Common mistakes and how to put them right

• Connecting an ammeter in parallel with a component.

  Put it right: An ammeter must be in series so the current passes through it.

• Connecting a voltmeter in series with a component.

  Put it right: A voltmeter must be in parallel with the component whose potential difference is measured.

• Thinking a variable resistor changes the potential difference of the supply directly.

  Put it right: It changes the total resistance, which changes the current and the sharing of potential difference.

• Leaving a test circuit permanently closed when not taking readings.

  Put it right: Use the switch to open the circuit and avoid overheating or draining the cell.

Exam tips

• When describing a test circuit, name each component and state whether it is in series or parallel and why.
• Link the design feature to its purpose: for example, the variable resistor controls current so that multiple readings can be taken.
• Use circuit symbols correctly in diagrams and label the ammeter, voltmeter, component under test and variable resistor.
• If asked to explain testing, refer to taking pairs of current and potential difference readings to plot a characteristic graph.